An internal combustion engine for an automobile includes a catalyst as means for purifying an exhaust gas. Especially in an internal combustion engine (for example, a gasoline engine) which performs a stoichiometric operation, a catalyst having an oxygen occlusion function, for example, a three-way catalyst is used. As the method for diagnosing a deterioration state of a catalyst having such an oxygen occlusion function, a so-called Cmax method is known. A Cmax method is a method which measures the oxygen occlusion capacity (Cmax) of a catalyst and diagnoses the deterioration of the catalyst from the measurement result.
In a Cmax method, an active air-fuel ratio control is performed, which oscillates a target air-fuel ratio with stoichiometry as a center, and thereby, forcefully changes an air-fuel ratio of the exhaust gas flowing into a catalyst between a lean side and a rich side. FIG. 4 shows a change with time of an actual air-fuel ratio (actual A/F) upstream of the catalyst and a change with time of an output value of a sub O2 sensor disposed downstream of the catalyst in combination when the target air-fuel ratio is changed between 14.1 and 15.1 by active air-fuel ratio control. In the Cmax method, integration of the oxygen occlusion amount or oxygen desorption amount of the catalyst calculated by the following expression is performed, until the output value of the sub O2 sensor downstream of the catalyst changes to exceed a threshold value (0.5 V), after the air-fuel ratio upstream of the catalyst changes with implementation of active air-fuel ratio control.
Oxygen occlusion amount or desorption amount=coefficient×(present air-fuel ratio−stoichiometry)×fuel amount injection amount
The result of calculating the oxygen occlusion amount and the oxygen desorption amount a plurality of times respectively by the aforementioned method and taking the average of them is set as a Cmax. FIG. 4 shows a change with time of the oxygen occlusion amount with respect to the Cmax with the time base thereof matched with the other graphs.
Incidentally, as the structure of the exhaust system of an internal combustion engine, there is known the structure in which a plurality of cylinders are grouped into two cylinder groups, an exhaust system is provided for each of the cylinder groups, and the two exhaust systems are collected into one exhaust collecting pipe, as shown in, for example, Japanese Patent Laid-Open No. 2006-112251. Further, in the structure of such an exhaust system, there is known the structure in which a catalyst is disposed in the exhaust collecting pipe so that the exhaust gas exhausted from each of the cylinders is collectively treated with the catalyst of the exhaust collecting pipe. Further, in such a structure of the exhaust systems, there is known the structure in which an EGR device is provided in one of the exhaust systems, and the EGR gas taken out of the exhaust system is recirculated into the intake system of each of the cylinders.
What becomes a problem here is the case of the EGR device provided with a catalyst. Hereinafter, the catalyst disposed in the exhaust collecting pipe will be called a main catalyst, and the catalyst provided in the EGR device will be called an EGR catalyst, in the structure of the exhaust systems described above. The main catalyst is in charge of purifying the exhaust gas which is exhausted from each of the cylinders, and the main catalyst is also the target of deterioration diagnosis by the Cmax method. As the environment in which the deterioration diagnosis of the main catalyst is performed, both the situation in which the EGR device is stopped and the situation in which the EGR device is operating are conceivable, but the presence of the EGR catalyst has an influence on the diagnosis result, in more detail, the calculation result of the Cmax in the situation where the EGR device is stopped.
When the EGR device is stopped, more specifically, when the EGR valve is totally closed, the EGR gas is not recirculated into the intake system from the exhaust system. However, even if the EGR valve is totally closed, inflow and outflow of the exhaust gas occur between the exhaust system and the EGR pipe with the variation of the exhaust pressure, and thereby, inflow and outflow of the exhaust gas to and from the EGR catalyst occur. FIG. 5 shows the result of investigating how the turbine inflow gas amount (total exhaust gas amount) and the EGR catalyst gas amount (gas amount flowing to and from the EGR catalyst) when the EGR valve is totally closed change in accordance with the crank angle. The drawing shows that the inflow and outflow of the exhaust gas to and from the EGR catalyst is the phenomena which constantly occur when the EGR valve is totally closed.
Accordingly, the flow of the exhaust gas when the EGR valve is totally closed, which is shown by the block diagram, is as in FIG. 6. α in the drawing represents a ratio of gas breathing into the EGR pipe, that is, the ratio of the exhaust gas which flows in and from the exhaust system and the EGR pipe. Of all the exhaust gases, the gas which directly flows into the main catalyst (S/C catalyst in the drawing) is an exhaust gas of 1-α, and the exhaust gas of a temporarily enters the EGR catalyst from the exhaust system, and thereafter, flows out to the exhaust system again to flow into the main catalyst. The exhaust gas which enters the EGR catalyst is purified close to stoichiometry in accordance with the oxygen occlusion amount of the EGR catalyst. Therefore, the purified exhaust gas of a and the unpurified exhaust gas of 1-α are mixed and flow into the main catalyst.
Meanwhile, FIG. 7 shows the flow of the exhaust gas when the EGR device is operating and the EGR is performed. In this case, when the EGR rate is set as β, the exhaust gas of 1-β out of all the exhaust gases flows into the main catalyst. The remaining exhaust gas of β flows into the EGR catalyst, and is recirculated into the intake system after passing through the EGR catalyst. Accordingly, in this case, the exhaust gas which is purified by the EGR catalyst is not mixed into the exhaust gas which flows into the main catalyst.
Of the two cases shown in FIGS. 6 and 7, a problem occurs in diagnosis of deterioration of the main catalyst in the case shown in FIG. 6. In the case shown in FIG. 7, the air-fuel ratio of the exhaust gas which flows into the main catalyst is not influenced by the EGR catalyst, and therefore, the air-fuel ratio of the exhaust gas which flows into the main catalyst can be controlled as intended by the active air-fuel ratio control. However, in the case shown in FIG. 6, the EGR catalyst functions as a low-pass filter when the target air-fuel ratio is oscillated at a high frequency by the active air-fuel ratio control. Therefore, it is difficult to control the air-fuel ratio of the exhaust gas which flows into the main catalyst as intended.
FIG. 8 shows the change of the target air-fuel ratio (target A/F), the change with time of the actual air-fuel ratio (actual A/F) upstream of the main catalyst, and the change with time of the output value of the sub O2 sensor disposed downstream of the main catalyst when the active air-fuel ratio control is performed in the case shown in FIG. 6, together with the change with time of the virtual actual A/F when the EGR catalyst is assumed to be absent. Further, FIG. 8 shows the change with time of the oxygen occlusion amount with respect to Cmax for the main catalyst and the EGR catalyst with the time axis matched with the other graphs. From this drawing, it can be read that the value of the actual A/F upstream of the main catalyst changes in accordance with the oxygen occlusion amount of the EGR catalyst. Further, the EGR catalyst is generally enhanced in oxidization reaction for its function, and therefore, a lean gas is purified faster as compared with a rich gas. Therefore, the time constants at the time of reversal of rich and lean of the air-fuel ratio of the exhaust gas which flows into the catalyst differ, and the time variation easily occurs in the desorption and occlusion of oxygen. Accordingly, it is found out that in the case shown in FIG. 6, the variations of the respective integration values of the oxygen occlusion amount and the oxygen desorption amount are large, and ensuring the estimation accuracy of the Cmax is difficult.
Further, when the air-fuel ratio of the exhaust gas which flows into the EGR catalyst changes in oscillation, the oxidization reaction on the catalyst is promoted. EGR is generally taken out from the place where the exhaust gas temperature is high (for example, upstream of the turbine), and therefore, depending on the degree of the oxidization reaction, the temperature of the EGR catalyst is likely to exceed the upper limit temperature. Therefore, the amplitude and the frequency in the active air-fuel ratio control are limited from the viewpoint of the upper limit temperature of the EGR catalyst, and due to the limitation, the deterioration diagnosis sometimes cannot be reliably performed.